The onset and completion of mitosis are universally controlled by activation and inactivation of the p34cdc2 protein kinase. Molecular studies have shown that p34cdc2 kinase activity is regulated by phosphorylation at multiple sites, and by means of a physical association with B-type cyclins. Physiological studies have demonstrated that cytoplasmic Ca2+ levels increase transiently at specific stages of the cell cycle, and that manipulations of cytoplasmic Ca2+ can accelerate or delay entry and exit from mitosis. To develop a more complete understanding of mitotic regulation at the cellular level, we have focused on defining the mechanism by which cytoplasmic Ca2+ transients may control p34cdc2 activation and inactivation. We previously established a link between Ca2+ and p34cdc2 by directly demonstrating that physiological levels of Ca2+ induce premature inactivation of the p34cdc2 kinase in permeabilized sea urchin embryos. We now show that activators of protein kinase C block p34cdc2 activation and inactivation in vivo by suppressing cytoplasmic Ca2+ transients which are required for the dephosphorylation of p34cdc2 at key regulatory sites. Our findings suggest a model for mitotic regulation in the sea urchin embryo whereby the association of p34cdc2 with cyclin B induces phosphorylation of p34cdc2 at activating and inhibitory sites, while cytoplasmic Ca2+ transients trigger the dephosphorylation of these sites that directly controls p34cdc2 protein kinase activity.